This is a continuing project aimed at elucidating the mechanism of action of opiate narcotics at the cellular, biochemical, and molecular levels. The knowledge gained may be expected to lead to an understanding of what biochemical changes underlie tolerance and physical dependence. Primary focus is upon isolation of opiate receptors from mouse brain and other neural tissues, and upon photolytic affinity labeling of opiate receptors in neurons of the myentric plexus of nontolerant and tolerant guinea pigs.